"BMW Z3: Air Conditioning (AC) System Operating Overview"

27 January 2017

Summary: The Z3 air conditioning systems has many components but if not functioning, worth a do-it-yourself check to ensure it is not something you can fix. Major culprits are: auxiliary radiator fan dead and system low on coolant.

Background:

Have a 1997, 2.8L, Z3 with 61,000 miles. Bought new.

Had a 2000 BMW X5, 4.4i, bought new but stolen and engine blown.

Have maintained both BMW's since 2005.

Am a college trained electrical engineer.

Overview of AC components:

The AC system on the Z3 consists of:

(1) A compressor unit driven by its own fan belt off the crankshaft. Standing in front of the Z3, on left hand drive models, the compressor is on the left side, just below windshield washer tank. Compressor units do go bad but should last 20+ years.

(2) Air exchanger coils, with its own blower, in dash. Because cabin temperature can get very high in the summer, these coils are exposed to extreme expansion and contraction and will eventually leak coolant. The blower in the dash can also go bad.

(3) A round dryer unit. This unit is located on the left side of the car, standing in front. A round metal cylinder with tubing into and out of it. The intention of the dryer is to remove any moisture than gets into system.

(4) Auxiliary radiator fan, electrically driven, located in front of the radiator. This fan helps keep the engine from overheating due to the added load of the compressor. When the compressor is running, the auxiliary fan is running at low speed. There is a temperature sensor installed into the radiator that detects radiator temperature above 190F or ? and switches the auxiliary fan to high speed mode. This switch can and does go bad.

(5) AC pushbutton and air temperature controls in cabin.

(6) Fuses for the compressor and associated blower unit. Located in fuse box under hood on right side, standing in front of Z3.

(7) Relays for the compressor and auxiliary fan (Actually 2: low speed and high speed). Located in fuse box, under hood, on the right side, standing in front of the Z3.

(8) A magnetic "clutch" on the compressor. When AC is "off", the fan belt on the front of the compressor turns but does not actually turn compressor shaft. When AC is turned "on", 12 volts is applied to the magnetic coil in the front of the compressor, making the fan belt actually turn the shaft of the compressor.

(9) Tubing between compressor, dryer and dash-based coils. There are coolant access ports both on the low pressure (gas) and high pressure (liquid) tubing. These ports have plastic caps over the end of them.

(10) "Freon" 134a. When compressed, 134a gives off heat. From compressed, if allowed to expand, absorbs heat. The compressor, compresses, the exchanger coils in the cabin dash absorb cabin heat.

Overview of AC system, workings:

When the cabin AC pushbutton is pushed in, and air temp control is turned to cold, the pushbutton. supplies 12 volts to a pressure sensor in the dryer and if sufficient 134a "Freon" pressure, the compressor relay is supplied power, engaging the magnetic clutch on the compressor. With the magnetic clutch engaged, the compressor begins to compress 134a from a gas into a liquid.

At the same time, the low speed auxiliary fan relay is closed supplying voltage to the auxiliary fan on the front of the radiator.

As the compressor runs, it compresses 134a and pumps 134a in the form of a liquid to dryer unit.

From the dryer unit, liquid is pumped to the coils in the dash and the auxiliary fan there is turned "on".

As 134a expands through the coils in the dash, the coils absorb heat or gets cold and the blower unit pushes cabin air across the cold coils.

When the temperature set by the cabin temperature control is reached, the compressor relay is opened, turning off the compressor.